A constitutional clash over whether House members are immune from many forms of Justice Department scrutiny has helped derail or slow several recent corruption investigations of lawmakers, according to court documents and sources.

At issue is a provision in the Constitution known as the "speech or debate" clause, which shields legislative work from executive branch interference. House members have increasingly asserted the privilege in corruption probes, often citing a 2007 court ruling that said FBI agents violated the Constitution when they searched the office of then-Rep. William J. Jefferson (D-La.).

The Justice Department warned at the time that the court decision would "seriously and perhaps even fatally" undermine congressional corruption probes by limiting the FBI's ability to search for evidence and use wiretaps.

Since then, speech or debate challenges have killed an investigation of former representative Tom Feeney (R-Fla.), hampered probes of Rep. Peter J. Visclosky (D-Ind.) and former representative John T. Doolittle (R-Calif.), and slowed a pending corruption case against former representative Rick Renzi (R-Ariz.), sources familiar with those inquiries said.

The widening assertions of criminal immunity have drawn little notice or controversy on Capitol Hill. In fact, the challenges have produced rare bipartisan agreement: The incoming and outgoing House leaders, for example, have jointly cited the constitutional clause to challenge much of the indictment of Renzi, who was extensively wiretapped. He is charged with attempting to benefit financially from a land deal. A brief filed by House lawyers compares Justice Department tactics in the case to illicit wiretapping under former FBI director J. Edgar Hoover.

House lawyers, who have aggressively countered the Justice Department in a number of investigations, have tried for two years to get the Obama administration to agree to limitations in Capitol Hill searches. But sources familiar with the negotiations said the talks broke down in the fall.

The speech or debate clause derives from 17th-century English law and says that "for any speech or debate in either House, (members of Congress) shall not be questioned in any other place." Until recently, courts had interpreted the passage narrowly.

"Bringing cases now because of the state of affairs of speech or debate makes these cases much more difficult," said Lanny A. Breuer, the assistant attorney general who oversees the Justice Department's Public Integrity Section. But he said that the provision "is not a roadblock. If you're corrupt, we're going to look at you, and we're going to prosecute you aggressively."

In all, the government has abandoned without filing charges at least seven investigations of current or former members of Congress since President Obama took office, many in recent months. They include the inquiries involving Feeney and Doolittle and others that ended for reasons apparently unrelated to speech or debate.

The other dropped investigations were examining Sen. John Ensign (R-Nev.), Rep. Don Young (R-Alaska), Rep. Jerry Lewis (R-Calif.), former representative Alan B. Mollohan (D-W.Va) and former House majority leaderTom DeLay (R-Tex.). Although the federal probe of DeLay was dropped, he was sentenced last week to three years in prison in a separate, state money-laundering case.

Law enforcement officials declined to comment on specific cases but said probes were dropped for a variety of reasons. They cited other court rulings, statutes of limitation and a review by new Justice Department officials who decided that some cases were not progressing.

But lawyers familiar with the issue said speech or debate concerns come up in virtually every congressional probe because many involve legislative acts, such as whether the lawmaker obtained earmarks in exchange for campaign contributions.

"The essence of speech or debate is you can't introduce into evidence what the member did if it's part of an official duty," said a lawyer who spoke on the condition of anonymity because the challenges are often under court seal. "If you can't introduce legislation, a bill, a speech on the floor, how do you make the case?''

In the Visclosky investigation, prosecutors explored whether the congressman helped obtain earmarks for clients of powerful lobbying firms in exchange for contributions. But Visclosky's lawyers refused to hand over much of the material sought in 2009 by a federal grand jury in Alexandria, citing speech or debate concerns, according to sources familiar with the investigation.

The investigation has not formally ended, but the sources said Visclosky is unlikely to be charged. Justice Department officials declined to comment.

The dropped investigations covered subjects ranging from Ensign's alleged efforts to keep secret his affair with a staff member to a $10 million earmark backed by Young for a Florida road project.

Mollohan announced last year that he will not be charged in a probe of his finances and nonprofit groups he created and helped fund in his district.

In an interview, Mollohan said the investigation was politically motivated. In each of the other cases, either the exonerated lawmaker or his attorney has disputed the allegations.

"I think it's a bum rap for people who don't know the evidence to assume there is something improper when career prosecutors determine they're not going to indict somebody," said Richard Cullen, a former U.S. attorney in Virginia who is chairman of McGuireWoods in Richmond. He represents DeLay, who announced in August that the Justice Department had dropped its six-year probe of his ties to former lobbyist Jack Abramoff.

Sources familiar with the federal investigation said that DeLay did not hold back documents from prosecutors but that they were hampered by their inability to persuade a key witness to testify against him.

Attorneys for Renzi, whose case has been delayed, have cited the speech or debate issue in their argument that the government violated his rights when agents wiretapped his cellphone, interviewed congressional aides without his consent and procured documents that aides took from his office.

Renzi's attorneys received support in June from the House general counsel, who argued in a brief on behalf of Speaker John A. Boehner (R-Ohio), Minority Leader Nancy Pelosi (D-Calif.) and other leaders that the Justice Department "repeatedly and flagrantly violated" Renzi's constitutional rights.

In 2007, the U.S. Court of Appeals for the District of Columbia said FBI agents violated the speech or debate clause when they conducted an unprecedented search of Jefferson's Capitol Hill office. Executive branch agents cannot review privileged legislative materials without the the consent of the member of Congress, a three-judge panel ruled.

Alarmed prosecutors urged the Supreme Court to overturn the decision.

"Its effect is to critically undermine the Executive Branch's ability to investigate and prosecute corrupt activity in and affecting the Legislative Branch," Justice Department lawyers wrote in a December 2007 brief. They said that vital investigative techniques could be compromised and that they might be forced to give lawmakers notice before the FBI searched their homes, vehicles or briefcases.

The Supreme Court declined to hear the case, which had the effect of making the restrictions law in the District.

Jefferson was nevertheless convicted of corruption in a case made famous by $90,000 found in his freezer.

Despite decisions to drop cases, Justice Department and FBI officials say they are as aggressive as ever in confronting congressional and other public corruption. They say they are increasingly focused on corruption nationwide and cite an influx of FBI agents to fight corruption along the nation's Southwest border and the recent indictment of 11 people, including four current and former state lawmakers, in a bribery inquiry related to bingo legislation in Alabama.

During the past two weeks, a federal grand jury in Richmond indicted a former member of Virginia's House of Delegates, Phillip A. Hamilton, on bribery and extortion charges, and Paul J. Magliocchetti, a former D.C. lobbyist, was sentenced to 27 months in prison for illegal campaign contributions.

"They may not be the sort of cases that will make headlines in Washington, but I think it's the sort of thing the American public wants us to do," said Jack Smith, who became chief of the Justice Department's Public Integrity Section in May.

A review by Smith and other officials of hundreds of public corruption investigations was a key factor in the decisions to drop some of the inquiries on Capitol Hill. Smith thought some investigations had lagged too long without sufficient progress, officials said.

Watchdog groups have criticized House leaders for their stance on speech or debate. The House brief in the Renzi case "was outrageous," said Melanie Sloan, executive director of Citizens for Responsibility and Ethics in Washington. "They're basically saying that if you are a member of Congress, you cannot be wiretapped under any circumstances."

Irvin Nathan, who resigned this month as House counsel, countered that prosecutors can use evidence collected through wiretaps or other means but must exclude anything related to legislative actions.

Speech or debate "does make life harder for the Justice Department, but that's what the framers intended," Nathan said.

Shortly after the court ruling on the search of Jefferson's office, attorneys for Feeney, under investigation in connection with a golf trip to Scotland financed by Abramoff, filed a challenge before a D.C. federal judge in which they cited the Jefferson decision.

That prompted the D.C. Circuit Court to throw out evidence, sought by a federal grand jury, from Feeney's statements to the House ethics committee.

The speech or debate challenge "was absolutely the reason" the Feeney investigation was dropped in 2009, a source familiar with the probe said, speaking on the condition of anonymity because the information is not public.

Doolittle, under investigation for his ties to Abramoff, also mounted a defense based in part on speech or debate.

Sources familiar with the investigation said Doolittle challenged a search of his Virginia home, along with a Justice Department subpoena for records. The sources said the battle hampered the investigation, but they would not comment further, citing sealed court proceedings.

And several years ago, the House general counsel's office challenged the subpoena of a House appropriations panel staff member in the probe of a lobbying firm's ties to Lewis. Sources said prosecutors wound up interviewing the staffer anyway.

The U.S. attorney's office in Los Angeles said last month that the Lewis investigation had been closed.

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